Public in private manifest errors (#16002)

### What does this PR try to resolve?

This is a stab at #13096, adding Cargo.toml context to public-in-private
errors. It intercepts public-in-private warnings from rustc and modifies
them by adding a little context.

I had a bit of difficulty with renamed crates (like, `foo_renamed = {
version = "0.1.0", package = "foo" }`). Surprisingly for me, rustc's
errors mention the original package name (foo) and not the renamed one
(foo_renamed), even though the renamed one is that one that's guaranteed
unique. This PR does its best to figure out which package is the one
being referred to, but if there's non-uniqueness involved then it just
gives up and doesn't provide context.

Author: Muscraft

src/cargo/core/compiler/build_context/target_info.rs

@@ -1105,6 +1105,10 @@ impl<'gctx> RustcTargetData<'gctx> {
         }
         unsupported
     }
+
+    pub fn requested_kinds(&self) -> &[CompileKind] {
+        &self.requested_kinds
+    }
 }
 
 /// Structure used to deal with Rustdoc fingerprinting